1. The effects of electrical pulse stimulation and temporary pharmacological inactivation of the ipsilateral cerebellar flocculus on the activity of single Y group cells were studied in three alert squirrel monkeys. The extent of the flocculus was mapped by multiunit recording and by electrical pulse train stimulation, which elicited slow eye movement. 2. Single electrical pulse stimulation of the flocculus (0.1-ms constant current, 25-400 microA) resulted in inhibition of all 24 Y cells examined. The inhibition was evidenced as a cessation of cell firing for varying periods [8.8 +/- 2.4 (SD) ms] after the stimulus. The latency of inhibition (0.71 +/- 0.34 ms) suggests that the effect was due to direct activation of Purkinje cells monosynaptically projecting to the Y group. 3. The gamma-aminobutyric acid (GABA) agonist muscimol was used to temporarily inactivate the flocculus while recording from single Y neurons. After control responses of cells under various behavioral paradigms were collected, muscimol (total volume of 3-4 microliters of 2.0% muscimol in saline) was injected in the flocculus through a pair of fine syringes. With this technique, the contribution of the flocculus to the signal content of Y group cells was examined, both in the animals with normal vestibuloocular reflex (VOR) gain (5 cells in 3 animals) and after adaptation of the VOR to either high (5 cells in 1 animal) or low gain (7 cells in 2 animals). 4. In the normal animal, pharmacological floccular inactivation resulted in increased dc firing and in the loss of normal modulation with eye velocity. Modulation during visual-vestibular interactions was also lost, so cell responses did not differ from those during the VOR in darkness. Only minor changes (usually gain increases) in the latter response were noted after flocculus inactivation. The results suggests an extrafloccular input source to the Y group, conveying head velocity information. We believe that this input originates in the brain stem, probably in the superior vestibular nucleus. 5. To examine whether the adapted responses of Y cells during the VOR in darkness are due to their floccular input, single cells were studied before and after pharmacological floccular inactivation, in animals whose VOR had been adapted.(ABSTRACT TRUNCATED AT 400 WORDS)